Semiconductor memory chips are formed as a plurality of dies on a wafer. Within each die, there are typically a large number of repeating structures (e.g. lines). In contrast, microprocessors formed on semiconductor wafers generally do not exhibit this type of repeating structures (except, for example, in certain regions allocated to microprocessor memory).
As device sizes continue to shrink, memory manufacturers are continuing to seek methods for measuring the uniformity of the repeating structures. A typical memory die could have dimensions in the range of about 120×120 microns (box size). A goal of the manufacturer is to obtain as many as 10,000 measurements within this box to determine uniformity of the film parameters and the structure geometry (critical dimensions). These measurements must evaluate uniformity on the scale of a micron or less.
One existing device for measuring optical properties of semiconductors is sold by Therma-Wave under the trademark Opti-Probe. This device includes many different non-destructive optical metrology technologies including Beam Profile Reflectometer (BPR), Beam Profile Ellipsometer (BPE), broadband reflectometry (BB) and spectroscopic ellipsometry (SE). BPR and BPE techniques are capable of measuring small spots (˜one micron or less) since they use a highly focused single wavelength laser output for a probe beam. In contrast, the BB and SE systems, which rely on a broadband source to generate the probe beam, have larger spot sizes, on the order of 15 microns.
None of these systems, if used alone, currently can provide the measurement results requested by the memory manufacturers. As can be appreciated, the BB and SE systems, while providing very accurate information, will produce an average measurement over their larger spot size. This average measurement does not have enough spatial resolution to satisfy the demands of the manufacturers. The BPR and BPE systems do provide sufficient resolution for measurement. However, the BPR and BPE systems produce a much smaller set of measurement data making the calculation of thin film and CD parameters quite difficult from these measurements alone. In addition, given the large number of requested measurement points, performing complex regressions for each of the measurement points would take a quite a long time, longer than would be acceptable to the manufacturers.
Therefore, it would be desirable to provide a measurement system which can rapidly obtain high-resolution measurements to permit evaluation of the uniformity of the semiconductor processing within small regions on the wafer.